What (default) tone mapping is applied to Cycles renders when saving to PNG?
The display transform for output files is determined in Color Management section. By default, blender uses a sRGB transform, but it can be changed.

(read: Render with a wider dynamic range in cycles to produce photorealistic looking images)

EXRs are an exception and are not processed using such display transform, so they store scene referred (linear float) data.



To bypass the display transform when saving on a display referred format, use the UV/Image editor, and uncheck "save as render"



See this post for more info: What does Save As Render do to the image when saving it?